1. Field of the Invention
This invention relates a placing method and a placing apparatus for placing balls with conductivity that are arranged in a predetermined pattern on a base unit, such as electronic parts or parts used for fabricating electronic parts etc.
2. Background Technology
Electronic parts like semiconductor devices, substrates or packages thereof with protruded connection bumps of area array type, such as BGA (Ball Grid Array) type and FC (Flip-Chip) type, etc., can be examples of the base unit. On the base unit, balls with conductivity (conductive balls, hereinafter) are placed in a predetermined pattern.
In recent years, as the portable machines and notebook computers progress in the direction of high speed, high performance, lightweight, small-size and thin shape, the performances of a large-dimension device and multiple terminals are required for the built-in electronic components. In responding to such requirements, the electronic components are adopted with the above area array type.
Methods for using a conductive material, such as a solder or copper, etc. to form connection bumps include a pasting method for printing paste, such as conductive material onto electrodes of the electronic components; conductive ball method for placing conductive balls on electrodes; and film attaching method for plating or depositing conductive material, etc. Due to trend of forming multiple terminals, the density of electrodes becomes high and the size of the connection bumps becomes correspondingly smaller. When forming small-size connection bumps, the conductive ball method is adapted for many cases because the conductive ball method is advantageous in alignment accuracy and productivity for the connection bumps.
According to the conventional conductive ball method, the connection bumps are formed by the following processes: a printing process for printing adhesive auxiliary such as solder paste or flux onto electrodes; a placing process for placing conductive balls onto the electrodes where the adhesive auxiliary has been printed; and a bump forming process for heating the conductive balls to form the connection bumps.
For the placing process, one way to place the conductive balls onto the electrodes, for example, is disclosed in Japanese Laid Open Publication 2001-223234 (Patent Document 1). As described in the publication, the method is based upon a suction mechanism such that the solder balls are held by a suction head using negative pressure and are transported to the electrodes to be placed thereon. However, the conductive balls might be deformed due to the suction force of the suction head. In addition, the conductive balls are held by the flux attached on the suction head, so that the conductive balls are not separated from the suction head even though the suction force is released. Therefore, an improper placement of the conductive balls may occur. Furthermore, the conductive solder balls moving in the air by the suction head will carry electrostatic charges. The collection of the solder balls aggregated by the electrostatic charges is placed on the electrodes, while the remaining solder balls (remaining balls or extra balls) may still be held onto the surface of the workpiece rather than the electrodes. This problem will be particularly obvious as the diameter of the conductive balls becomes smaller and smaller.
An alternative method for placing conductive balls is the so-called transfer method. In the transfer method, a plate-shaped mask is used as an arrangement member where throughhole-like positioning openings corresponding to the pattern of the electrodes are formed. The conductive balls supplied to the mask are loaded into the positioning openings (this operation is known as transfer), and are placed onto the electrodes through the positioning openings. One example of the transfer method is disclosed in Japanese Laid Open Publication No. 2002-171054 (patent document 2), Japanese Laid Open Publication No. H09-162533 (patent document 3), Japanese Laid Open Publication No. 2001-267731 (patent document 4) and Japanese Laid Open Publication No. H10-126046 (patent document 5).
The patent document 2 discloses a solder ball placement device for placing solder balls on fluxes that are applied to a plurality of locations on a workpiece surface. The solder ball placement device comprises a mask, a tilting mechanism and a restriction member. The mask is used to cover the workpiece and the mask has a plurality of ball-holding holes at positions corresponding to the positions of the plurality of fluxes, where the solder balls are capable of passing through these ball-holding holes. The tilting mechanism is used for tilting the workpiece and the mask. The restriction member is used to restrict the moving speed of the plurality of solder balls that moves from an upside to a downside on the surface of the mask in order for the solder balls to fall into the positioning openings. The falling conductive balls are restricted by the restriction member, and can be loaded into the positioning openings by moving the conductive balls at a proper speed.
The patent document 3 discloses a placement device, in which the bottom of a solder-supplying head is in contact with a solder-supplied object where the solder is to be supplied thereon. A solder-supplying unit is slid on the inner surface of the bottom of the solder-supplying head. A plurality of spherical solders previously supplied to the solder-supplying head is advanced to the inner surface of the bottom of the solder-supplying head. In this way, through the respective solder supplying holes, the spherical solders are respectively supplied to each of the solder positions of the solder-supplied object. A discharging brush is used as a transfer device (i.e., a solder supplying unit), and by sliding the erect front ends of the discharging brush, the conductive balls are respectively supplied to each of the solder positions of the solder-supplied object.
The patent document 4 discloses a placement method. The placement method comprises an adhesive film forming process for selectively forming adhesive films on the respective electrodes on an electronic component; a joint material arranging and supplying process for arranging and supplying a joint material on the respective adhesive films formed in the adhesive film forming process; and an affixing process for melting the joint material supplied in the joint material arranging and supplying process and then affixed to the electrodes. A ball arranging mask, i.e., an arrangement member, is disposed on a wafer. The solder balls are supplied onto the ball arranging mask. The solder balls are transported by dispensing device (i.e., a transfer device) and are placed on the electrodes on which the adhesive films are formed.
The patent document 5 discloses a placement device for placing tiny solder balls on a pattern formed on a substrate in a ball grid array device and a holding stage for holding the substrate. A mask having guiding holes corresponding to the pattern on the substrate for the solder balls is disposed above the substrate. A spacer is formed between the mask and the substrate. A blade capable of horizontally moving is disposed over the upper surface of the mask. Solder balls are disposed on the upper surface of the mask. Then, one by one, the solder balls fall into the guiding holes of the mask by the horizontally moving the flexible blade. The solder balls are scraped to be collected at one side of the mask, and only the required portions of the mask are placed with the solder balls.
In the transfer method, since the conductive balls are placed by gravity, it is advantageous that the problems occurred in the suction method can be solved. In addition, due to the restriction of the size of the position opening, the collection of the aggregated conductive balls is not placed on the electrodes even though the conductive balls are charged. Since the portions other than the electrodes are blocked by the mask, the remaining balls will not be placed on the undesired portions.                [Patent Document 1] Japanese Laid Open Publication 2001-223234        [Patent Document 2] Japanese Laid Open Publication 2002-171054        [Patent Document 3] Japanese Laid Open Publication He9-162533        [Patent Document 4] Japanese Laid Open Publication 2001-267731        [Patent Document 5] Japanese Laid Open Publication He10-126046        
Nowadays, the electronic components are required to have more terminals and be smaller in size. In this case, the number of the connection bumps increases tremendously, while the pitch becomes smaller and the density becomes higher. The diameter of the conductive ball has to be reduced to 100 μm or less. However, problems occur when the diameter of the conductive balls becomes smaller.
Regarding the transfer method, if the diameter of the conductive ball is small, the rate to fill the positioning opening decreases. Therefore, a portion of the electrodes is not placed with the conductive balls. Thus, the placement ratio is possibly reduced. The placement ratio is defined with the following formula: number of conductive balls placed on predetermined positions/number of predetermined positions to be placed. Specifically, for example, when the base unit has a plurality of electrodes, the placement ratio is defined as follow: the number of conductive balls placed on the electrodes/the number of the electrodes.
When the diameter and the mass of the conductive balls are large, the conductive balls that are loaded into the positioning openings fall with sufficient energy by their own weight to impact with the flux. The conductive balls are placed on the electrodes and are firmly adhered to the flux. The contact area between the large-diameter conductive ball and the flux is large. Therefore, the conductive balls that are firmly adhered and held by the flux are seldom separated from the electrodes even though they are being subjected to external forces.
However, if the conductive balls are small in diameter and mass, the conductive balls that are loaded into the positioning openings fall with small energy to impact with the flux. Further, the contact area with the flux also becomes smaller. As a result, the adhesive force between the conductive balls and the flux is small. Thus the conductive balls are easily separated from the electrodes even though they are only subjected to a small external force. For example, when a flexible dispenser is used as the transfer device to transfer the conductive balls to the positioning openings, the front end of the dispenser is disposed inside a portion of the positioning openings, and the conductive balls are scraped out by the front end to separate from the electrodes. Therefore, the placement ratio for the conductive balls reduces.
As the diameter becomes smaller, the conductive ball behaves like powders. Therefore, the domination of the inertial force (gravity) becomes relatively small. For example, since the electronic components are manufactured in an atmospheric environment, a portion of the mask used as the arrangement member will carry electrostatic charges or adhered with moisture. When the conductive balls are large in diameter and mass, the inertial force (gravity) dominates. When the conductive balls are transferred by the dispenser over the mask or the conductive balls are loaded into the positioning openings, the moving conductive balls have sufficient inertial force (gravity). Therefore, as described above, the adhesion of conductive balls to a portion of the mask due to electrostatic charges or water seldom occurs.
As the diameter and the mass of the conductive ball become smaller, the magnitude of the adhesive force due to static electricity or moisture would become large when compared with the inertia force (gravity). Therefore, as the conductive balls are adhered at the positioning openings by the suction force, conductive balls are not properly placed on the electrodes. As a result, the placement ratio reduces. In addition, as the conductive balls are adhered on the surface of the mask, the placement ratio of the conductive balls by a single dispensing operation is low. As a result, a plurality of transfer operations has to be performed. However, the plurality of transfer operations will promote a phenomenon in which the conductive balls filled in the positioning openings are scraped out by the front end of the dispenser. Therefore, it is difficult to improve the placement ratio. Furthermore, the conductive balls adhered to the mask might remain adhered to the mask. When removing the mask from the electronic component, the conductive balls adhered to the mask will be separated from the mask, which is the reason of short-circuit of the conductor circuits.
As shown in FIG. 20(a), the electronic parts has unavoidable deformations, such as curve and variation of the thickness, caused by the difference of the expansion rate between the electronic parts 7 and the electrodes 71 or due to the conductor pattern or the manufacturing conditions, etc. As the mask 92 is arranged one the electronic component 7, a gap between the electronic component 7 and the mask 92 is created. When the gap is larger than the conductive ball, the conductive balls loaded in the positioning openings 921 will escape through the gap and fail to be placed on the electrodes 71. As a result, the placement ratio reduces. In addition, the escaped conductive balls become redundant conductive balls, causing a short circuit of the conductive pattern. In addition, the escaped conductive balls may adhere onto the peripherals of the electrodes 71 and the plural of conductive balls may link to form a bridge.
When the mask is positioned on the electronic component, the flux being applied on the electrodes may adhere on the inner walls of the positioning openings. The conductive balls loaded into the positioning openings are captured by the flux attached on the inner walls. As a result, the placement ratio reduces. In addition, there might be a case that the flux is adhered on the mask and the mask can not be separated from the electronic component.
Regarding with the transfer method, the conductive balls which are more than the number of the electrodes are supplied to the upper surface of the mask to perform the transfer operation, and the redundant conductive balls that are not filled in the positioning openings are recycled. In most cases, the recycled conductive balls are used for the next transfer operation. During performance of the transfer operation, the flux might be adhered on the conductive balls. The conductive balls on which the flux is adhered can not be smoothly loaded into the positioning openings. As a result, the placement ratio reduces. Regarding with the flux issue, it can be solved by cleaning the mask and the conductive balls, but the manufacturing cost will increase.